Oil sand, such as is mined in the Fort McMurray region of Alberta, generally comprises water-wet sand grains held together by a matrix of viscous bitumen. It lends itself to liberation of the sand grains from the bitumen, preferably by slurrying the oil sand in hot process water, allowing the bitumen to move to the aqueous phase.
For many years, the bitumen in the McMurray sand has been commercially removed from oil sand using what is commonly referred to in the industry as the “hot water process”. In general terms, the hot water process involves the following steps:                dry mining the oil sand at a mine site that can be kilometers from an extraction plant;        conveying the as-mined oil sand on conveyer belts to the extraction plant;        feeding the oil sand into a rotating tumbler where it is mixed for a prescribed retention time (generally in the range of 2 to 4 minutes) with hot water (approximately 80-90° C.), steam, caustic (e.g., sodium hydroxide) and naturally entrained air to yield a slurry that has a temperature typically around 80° C. The bitumen matrix is heated and becomes less viscous. Chunks of oil sand are ablated or disintegrated. The released sand grains and separated bitumen flecks are dispersed in the water. To some extent bitumen flecks coalesce and grow in size. They may contact air bubbles and coat them to become aerated bitumen. The term used to describe this overall process in the tumbler is “conditioning”; and        diluting the slurry so produced with additional hot water to produce a diluted slurry having a temperature of about 65° C. to about 80° C. The diluted slurry is introduced into a large, open-topped, conical-bottomed, cylindrical vessel termed a primary separation vessel (PSV) where the more buoyant aerated bitumen rises to the surface and forms a froth layer. This froth layer overflows the top lip of the PSV and is received in a launder extending around the PSV's rim. The product is commonly called “primary froth” and typically has a temperature of about 65° C. to about 75° C.        
Throughout the years, the hot water process has been modified to be more energy efficient by reducing the process temperature and replacing short-duration, high-temperature conditioning in a tumbler with longer-duration, lower-temperature conditioning in a hydrotransport pipeline. A “warm slurry extraction process” was developed in the early 1990s and is disclosed in Canadian Patent No. 2,015,784. In the late 1990s, a cold dense slurrying process for extracting bitumen from oil sand was developed, which is disclosed in Canadian patent No. 2,217,623. This process is commonly referred to as the “low energy extraction process” or the “LEE process”.
All of the above extraction processes use a PSV for receiving diluted oil sand slurry and separating the aerated or floating bitumen from the sand. The typical residence time in the PSV is approximately 45 minutes. During this time, the sand sinks and is concentrated in the bottom section of the PSV where it is removed as tailings underflow (also referred to herein as “PSV tailings”). Some valuable bitumen is lost to this underflow. Further, a portion of the non-aerated (i.e., non-floating) bitumen remains in the watery mid-section of the PSV, together with dispersed fine solids. This watery suspension is referred to in the industry as “middlings” and in the present invention will also be referred to as “PSV middlings”.
To improve overall bitumen recovery, it is important to capture both the floating and non-floating bitumen that still remains in the middlings and tailings underflow. One way for recovering bitumen from PSV tailings and middlings is described in Canadian Patent No. 1,248,476 and U.S. Pat. No. 4,545,892. In these patents, the PSV tailings and middlings are combined and introduced into a second separation vessel referred to as the tailings oil recovery vessel (“TORV”). Briefly stated, the incoming feed to the vessel is deflected and spread out laterally and contacted from below by an upwelling stream of aerated and recycled TORV middlings. The air bubbles in the recycled TORV middlings contact and aerate previously non-buoyant bitumen in the feed to produce “secondary” bitumen froth.
More particularly, a plenum assembly is positioned inside the TORV, centrally of the chamber of the TORV, in the body of the TORV middlings. The plenum assembly comprises a plurality of eductor/aerator devices, each eductor/aerator device comprising a nozzle and a venturi tube, the nozzle being positioned close to but gapped from the venturi tube. Each eductor/aerator device extends outwardly from the plenum assembly lower wall, being connected to the lower wall by the venturi tube. Finally, each nozzle is circumscribed by a tubular sparger mounted thereon.
The recycled TORV middlings are upwardly fed at a high velocity into the nozzles of the eductor/aerator assemblies in order to entrain additional middlings present in the TORV. Pressurized air is fed to the tubular sparger to aerate the recycled middlings feed. The aerated middlings exit the nozzle and are injected into the inlet of the venturi tube. The motive jet of recycled middlings induces a flow of non-aerated middlings from the TORV chamber through the gap formed between the nozzle and the venturi tube. The aerated middlings first collect in the plenum chamber and are then discharged outwardly, slightly upwardly, and generally radially from the plenum chamber through slot-like outlets.
In operation, however, there were several drawbacks to using the above-described plenum assembly. For example, the eductor/aerator devices would routinely become plugged with or damaged by the solids present in the recycled middlings. Further, the recycled middlings had to be fed at relatively high motive velocities in order to generate additional entrainment of middlings, which further contributed to the erosion damage to the eductor/aerator devices. Finally, because the plenum assembly was situated inside the TORV, the entire TORV operation had to be shut down and the plenum assembly removed whenever maintenance of the plenum assembly was required. The TORV would have to be drained to enable access to the normally submerged assembly, resulting in labor and repair costs as well as lost production.
The present invention addresses some of the problems associated with the plenum assembly by providing a gravity separation vessel having mounted thereto a downpipe assembly comprising at least one downpipe having a nozzle for aerating recycled middlings, which downpipe assembly does not need to be internally mounted. Thus, the downpipe assembly is easily accessible for maintenance without having to shut down the entire vessel operation.